1. Field of the Invention
The present invention relates to a dielectric ceramic composition, and in particular, relates to a dielectric ceramic composition used as a material for a monolithic ceramic capacitor or the like.
2. Description of the Related Art
Conventionally, dielectric ceramic compositions in the field of the invention and in which the main component is BaTiO.sub.3 and the subcomponents are a bismuth compound, e.g., Bi.sub.2 O.sub.3 --TiO.sub.2, Bi.sub.2 O.sub.3 --SnO.sub.2, and Bi.sub.2 O.sub.3 --ZrO.sub.2, and a rare earth metal oxide, are widely known to exhibit a small temperature dependence of its characteristics such as dielectric constant, mechanical strength and stability of the dielectric constant over wide temperature ranges.
It is also reported that another dielectric ceramic composition of which the main component is BaTiO.sub.3 and the subcomponents are Nb.sub.2 O.sub.3, a rare metal oxide, and an oxide of a transition metal, e.g., Cr, Mn, Fe, Co and Ni, has a flat temperature characteristic of the dielectric constant while having the high dielectric constant of 3,000 or more.
The temperature characteristic of those dielectric ceramic compositions satisfy the X7R characteristic of the EIA standard, i.e. the percentage of capacitance change in a temperature range of -55.degree. C. to +125.degree. C. is limited within .+-.15% on the basis of the reference electrostatic capacitance at 25.degree. C.
Recently, ceramic monolithic capacitors have been applied to the EEC modules which electronically control an automobile's engine. These modules are implemented in an engine room in which, when the engine operates, the temperature ranges from approximately -20.degree. C. in winter to approximately +130.degree. C. in summer. Further, the temperature is likely to rise to approximately +150.degree. C. if the engine becomes overheated. Thus, conventional dielectric ceramic compositions satisfying only the X7R characteristic cannot be employed.
The dielectric ceramic composition in which the main component is BaTiO.sub.3 and the subcomponents are Nb.sub.2 O.sub.3, a rare metal oxide and an oxide of a transition metal, e.g., Cr, Mn, Fe, Co and Ni, exhibits relatively small mechanical strength. Moreover, the physical characteristics of these compositions, such as dielectric constant, largely depend on the applied voltage. Accordingly, small-sized ceramic monolithic capacitors with high capacitance cannot be attained using a thin film of the dielectric composition.
Further, although the dielectric ceramic compositions having BaTiO.sub.3 as a main component and a bismuth compound as a subcomponent exhibit low dependence of their physical characteristics on the applied electric voltage and a high mechanical strength, the dependence of the dielectric constant on temperature becomes larger when the dielectric constant of the composition is made to be high. Moreover, to use those dielectric ceramic compositions for monolithic ceramic capacitors, it is necessary to increase the amount of Pb in the material which forms internal electrodes because the required firing temperature is 1,160.degree. C. or more. Such internal electrodes are high in cost. In addition, the reaction between Pd and Bi.sub.2 O.sub.3 increases.